This guide assumes that you have already unlocked the bootloader
1- Download latest version of TWRP for Nextus 5X
2- Restart to bootloader
$ adb reboot-bootloader
3- Flash TWRP recovery image
$ fastboot flash recovery twrp-3.0.0-1-bullhead.img
4- Download latest version of SuperSu stable
5- Boot to recovery mode by select the recovery using the volume buttoms
6- Copy the download zipfile to the phone
$ adb push UPDATE-SuperSU-v2.65-20151226141550.zip /sdcard/Download/supersu.zip
7- Go to install menu on TWRP, select the supersu.zip file and swipe to confirm flash
8- Once you've installed SuperSU you'll have an option to wipe cache/dalvik and an option to reboot system. Wipe the cache/dalvik, hit the back button, and hit the reboot system button. That's it.
Done!
Unrooting is needed for installing OTA updates.
1- Download latest build image from here: https://developers.google.com/android/nexus/images#instructions
- Make sure to find the current build number from settings > about
2- Unzip the file
3- Restart to bootloader (USB Debugging should be enabled)
$ adb reboot-bootloader
4- Run the following commands sequentially (You have to change to path/to to the correct path and update the filenames). All the files used in these commands are found in the build image downloaded in step 1
$ fastboot flash bootloader bootloader-bullhead-bhz10k.img
$ fastboot reboot-bootloader
$ fastboot flash radio radio-bullhead-m8994f-2.6.30.0.68.img
$ fastboot reboot-bootloader
$ fastboot flash boot image-bullhead-mmb29q/boot.img
$ fastboot erase cache
$ fastboot flash cache image-bullhead-mmb29q/cache.img
$ fastboot flash recovery image-bullhead-mmb29q/recovery.img
$ fastboot flash system image-bullhead-mmb29q/system.img
$ fastboot flash vendor image-bullhead-mmb29q/vendor.img
5- Restart the device